It is known in the prior art to manufacture MEMS devices. In particular, devices such as accelerometers and gyro sensors are made with a hermetically sealed chamber in which the movable MEMS member is disposed. The hermetic seal protects the MEMS device from the environment. For example, the hermetic seal may protect the device from dust, moisture and dirt. Moreover, changes in air pressure within the MEMS chamber will cause changes in the sensitivity of the MEMS sensor.
Economical manufacture of mass quantities of MEMS devices generally involves simultaneously forming multiple MEMS devices on a semiconductor wafer. When there is a need for a hermetic seal, a manufacturer charged with producing such MEMS devices needs to be able to test each device to assure that a reliable seal has been formed during the processing. Resonant frequency testing determines for each device a measure of the resonant frequency and a measure of quality factor (Q). U.S. Pat. No. 7,739,900 (Reinert et al.) describes bombing vacuum bonded MEMS devices being tested with a high pressure of 1.5 to 5 bars for a period of 10 to 100 hours. Q factor measurements are used to determine which devices are leaky.
For devices capped to maintain the pressures in their chambers significantly away from one atmosphere, resonant frequency testing at atmospheric pressure can detect the leaky devices. For devices bonded in a vacuum, the hermetically sealed devices will exhibit a Q factor above that of devices that leak. Differences can be recognized because of the sensitivity of Q factor to changes in pressure at the low pressures inside the MEMS chambers. To make the test sensitive enough to readily distinguish leaky devices from hermetically sealed devices, the pressure within the chamber of the hermetically sealed devices should be as far below 1 atmosphere as possible.